Give a small cheer to the index nerds at S&P. Their decision to ban companies that have different classes of stock is a rare instance of Wall Street protecting investors.

S&P said Monday that it would no longer consider companies with multiple share classes for its main U.S. stock indexes. The one that matters is the S&P 500, which is tracked by about $2.2 trillion worth of assets and which serves as a benchmark for more than $7.8 trillion of investments. The share structures S&P is targeting usually grant...